It is common to use either three or four springs to support a shadow mask within a rectangular faceplate panel of a color picture tube. In a three spring support system, one spring is usually located at the upper center of the mask, and the other two springs are located along the sides of the tube between the centers of the sides of the mask and the lower two corners of the mask. In a four-spring support system, springs are usually located at the top and bottom centers of the mask and at the left and right centers of the mask. In both the three- and four-spring support systems, as described above, it is possible for the shadow mask to slightly twist and shift relative to the faceplate during manufacture and tube operation.
One means for minimizing twisting and shifting of a shadow-mask uses spring supports located at the four corners of the frame. Embodiments for achieving such corner support are shown in U.S. Pat. No. 4,723,088, issued to Sone et al. on Feb. 2, 1988, and in U.S. Pat. No. 4,728,853, issued to Sone et al. on Mar. 2, 1988.
An improvement in tubes having corner supports is disclosed in U.S. Pat. No. 5,021,707, issued to Bauder on Jun. 4, 1991. Unlike previous tubes, which aligned the springs and support studs with the tube faceplate panel diagonals, the Bauder patent discloses aligning the springs and studs off of the diagonals so that the support axes form an angle of 33 to 50 degrees with the major axis of the panel. The purpose of offsetting the axes of the supports from the diagonals is to improve the resistance of the supports to shock.
A problem is encountered with having the support axes offset from the diagonals. This problem occurs when the maskframe assembly and support means heat up, and the expansion of the assembly causes the support springs to re-seat on the studs. The problem is especially severe during tube processing, when the temperature of the tube can reach approximately 450.degree. C. Because such re-seating may cause an electron beam misregister, it is desirable to solve the problem. The present invention provides such a solution.